The proposed work is concerned with the biophysical and biochemical characterization of physiological parameters of photoreceptors. The biophysical studies include the analysis of light-induced current fluctuations recorded from the ventral photoreceptor of Limulus under voltage-clamp conditions. It has been shown that these fluctuations can be related to the underlying discrete conductance channels. Properties of these channels are also being studied by using noise analysis. The biochemical studies include the characterization of proteins in Drosophila photoreceptors by the 2-dimensional gel electrophoresis technique. With such 2-dimensional gels from wild type and mutants with known defects in the phototransduction process, insights into the proteins involved in the process can be gained. Ultimately, properties of the proteins should serve as the molecular basis for the biophysical characterization.